1. Field of the Invention
The present invention relates generally to gas turbine combustors and more specifically to a gas turbine combustor that allows the same burner to burn two kinds of fuel gases with different heating values.
2. Description of Related Art
In recent years, the beneficial use of blast furnace gas (BFG) and coke oven gas (COG) co-produced in steel plants has been examined from a viewpoint of a reduction in power generation cost, the beneficial use of resources and the prevention of global warming. Blast furnace gas is produced in steel production process and is flame retardation gas containing carbon monoxide and hydrogen as main flammable gas. In addition, the blast furnace gas is a so-called low Btu gas having a heating value of about 1000 kcal/m3N. Therefore, it is difficult to stably operate the gas turbine through blast furnace gas mono-firing over a period ranging from ignition to a full load operation. To stably operate the gas turbine for combustion over a range from the ignition to a partial load with low combustion temperature, it is necessary to mix coke oven gas containing hydrogen with blast furnace gas to increase a heating value for operation (carburetion), or to separately provide start-up fuel such as liquid fuel.
On the other hand, the coke oven gas is off gas that is produced when coke, which is the raw material for the blast furnace, is produced. In addition, the coke oven gas is medium Btu gas, which contains hydrogen and methane as major composition and has a heating value of 4000 kcal/m3N to 5000 kcal/m3N. Containing hydrogen, the coke oven gas has a heating value higher than that of the blast furnace gas. Therefore, the coke oven gas is used as a carburetion gas for blast furnace gas firing gas turbines, or as a main fuel for coke oven gas firing gas turbines.
To stably burn low Btu gas such as BFG, a gas turbine combustor is provided that includes a start-up oil nozzle located at the radially central portion of a burner, an inner swirler having gas holes arranged on the outer circumference thereof, and an outer swirler in which gas holes and air holes are alternately arranged on the outer circumference of the inner swirler (see JP-5-86902-A).
In general, burners stabilize flames using swirl flows. In order to stabilize flames, such a burner need to have a recirculation zone formed in the vicinity of the radially central portion of the burner. The recirculation zone is applied to circulate combustion gas and convey heat to the fuel and air jetted from the burner.
According to the gas turbine combustor described in JP-5-86902-A, only gas holes are arranged in the inner swirler and most of fuel is supplied to the gas holes. By doing so, the kinetic momentum based on a large amount of low Btu gas is utilized to form strong swirl flows, from which the flame stabilization is strengthened. Fuel jetted from the inner swirler is taken in the recirculation zone while mixed with air jetted from the outer swirler, so that oxygen (concentration) in the recirculation zone will suffice. Thus, the stable combustion of low Btu gas is possible.